1. Field of the Invention
This invention relates to an oscillation circuit of the multiphase coupled ring type and a recording apparatus wherein a multiphase clock is oscillated.
2. Description of the Related Art
Presently, a multiphase clock is used widely for timing regeneration, frequency/phase modulation and demodulation, delay measurement, write clock generation or write strategy on an optical disc represented by a DVD (Digital Versatile Disc) and a BD (Blu-ray Disk), and so forth.
For example, a write clock signal for an optical disk requires generation of a special write waveform and requires a phase resolution of such a high degree of accuracy of one several tenth the writing data rate.
Such a write clock as just described is generally implemented by selecting phases of a multistage ring oscillator OSC formed from a plurality of delay elements DLE connected in a ring form as seen in FIG. 1A by means of such a selection circuit MUX as shown in FIG. 1B and combining the selected phases.
Therefore, it is demanded for the multistage ring oscillator not only to have a high phase resolution but also to be accurate in regard to the phase order and position.
In recent years, the data rate for writing on an optical disk has become higher, and it becomes necessary for a multistage ring oscillator to have a phase resolution higher than an oscillation frequency.
In order to achieve such a high phase resolution as just mentioned, in the multistage ring oscillator described above with reference to FIG. 1A, it is necessary to suppress the delay time of the delay elements DLE per one stage to short time.
In other words, it is difficult to implement such a high phase resolution as described above from a point of view that very high current consumption is required.
Therefore, such a multistage coupled ring oscillation circuit as shown in FIG. 2A has been proposed in Japanese Patent Laid-Open No. 2000-156629 (hereinafter referred to as Patent Document 1), JP-T-2008-529318 (hereinafter referred to as Patent Document 2), J. G. Maneatis and M. A. Horowitz, “Precise delay generation using coupled oscillators,” IEEE J. Solid-State Circuits, Vol. 28, No. 12, pp. 1273-1282, December 1993 (hereinafter referred to as Non-Patent Document 1) and A. Matsumoto, S. Sakiyama, Y. Tokunaga, T. Morie and S. Dosho, “A Design Method and Developments of a Low-Power and High-Resolution Multiphase Generation System,” IEEE J. Solid-State Circuits, Vol. 43, No. 4, pp. 831-843, April 2008 (hereinafter referred to Non-Patent Document 2). A high-speed fine multistage clock can be implemented by the coupled ring oscillator.
FIGS. 2A to 2C show an example of a configuration of a multistage coupled ring oscillation circuit.
The multiphase coupled ring oscillation circuit 10 shown in FIG. 2A has a configuration for nine phases.
The multiphase coupled ring oscillation circuit 10 is configured such that a plurality of (three) small-number-stage (three-stage) inverter ring oscillation circuits 11 to 13 shown in FIG. 2B are coupled by a single multistage (nine-stage) inverter ring circuit 14 shown in FIG. 2C.
In other words, each two small-number-stage rings are coupled by a single phase coupled element (path) and such three rings are coupled into a ring.
In the following description, each of the inverter ring oscillation circuits 11 to 13 shown in FIG. 2B is referred to as main ring, and the inverter ring oscillation circuit 14 shown in FIG. 2C is referred to as phase coupled ring.
In FIG. 2A, a phase coupled element of the phase coupled ring 14 is indicated by an inverter INV.
However, as disclosed in Patent Document 1, Patent Document 2 and Non-Patent Document 2 mentioned hereinabove, the phase coupled element may not be an inverter but may be any element which can determine the phase between main rings such as, for example, a resistor.
In particular, the phase coupled ring itself needs not oscillate but is driven by the plural main rings to oscillate.
In other words, the oscillation frequency is determined by the small-number-stage main rings 11 to 13, and the phase between the main rings is determined by the phase coupled ring 14.
In short, a high-speed and fine multiphase clock which is difficult to implement using an ordinary multistage ring oscillation circuit or a like circuit can be implemented by the multistage coupled ring oscillation circuit.